ATMOSPHERIC ABSORPTION AND SCATTERING 



149 



li meter per hour rate of rainfall, at 1.25-, '.]-, 5-, 8-, and 

 10-em wavelengths, respectively. The drops forming 

 these rains were supposed to he at temperatures near 

 18 C. By increasing the preceding values hy ahout 30 

 per cent one would very likely take care of raindrops at 

 lower temperatures, since the absorption increases 

 with decreasing temperature of the drops. 



In the computation of attenuation for rains it was 

 assumed that ideal conditions prevailed throughout the 

 rains under consideration. By this was meant that the 

 same sample of rain falling, sa}', over an area of 1 sq m 

 would be found anywhere inside the area covered by 

 the rain. Sucli ideal rains seem to be rather simple 

 theoretical models. Considerable fluctuations in the 

 rate of rainfall o^•er relatively short distances (1 km 



100.0 



20.0 



10.0 



5.0 





1- 

 a 

 o: 

 o 

 o> 



2.0 

 1.0 

 .5 



.2 



.1 



.05 



.02 



.01 



.005 



.002 



.001 



,0005 



,0002 

 ,0001 



3.0 6.0 9.0 15 30 60 

 FREQUENCY IN lO' MC 



90 150 



5 4 



1.5 ) 

 X IN CM 



-| — 

 0.8 



— 1 — I — I r 



0.5 0.4 03 02 



Figure 1. Oxygen and water vapor absorption versus 

 wavelength. (1) Absorption due to water vapor in an 

 atmosphere at 76-cm pressure containing 1 per cent 

 water molecules, or 7. .5 g ])er cu m. The water resonance 

 line is assumed to be at 24,000 mc, and its half-width at 

 half maximum (line breadth) is 3,000 mc. (2) Absorp- 

 tion due to oxygen in an atmosphere at 76-cm pressure, 

 whose resonance band at 60 • 10^ mc is supposed to have 

 a line breadth of 600 mc. 



or less) have been reported.^" These spatial irregular- 

 ities of rains exclude any simple interpretation of the 

 experimental data on rain attenuations. The computed 

 values of attenuation are based on a few data on drop 

 size distributions" in rains. 



In Figure 1," the individual oxygen and water 

 vapor atteiruation curves have been plotted in the 0.2- 

 to 10-cm wavelength range, using the most acceptable 

 data available on the position of line centers and line 

 widths. Airy change in the water vapor content from 

 the one adopted for this graph (7.5 g/m^ of air or 6.2 

 g per kilogram of air) or the total pressure can be 

 taken rapidly into account in computing the combined 

 oxygen and water vapor attenuations, since the atten- 

 luition \'alues are proportional to the partial pressures 

 of oxygen and water vapor. For practical purposes the 

 effect of atmospheric temperature variations can be 

 neglected. 



In Figure 3 is plotted the total (oxygen plus water 

 vapor) attenuation (curve 1) in an atmosphere at 

 76-cm pressure with the same water vapor content 

 as the water curve of Figure 1. Curves 3, 3, and 4 are 

 rain attenuation curves computed for a moderate rain 

 (rate of rainfall 6mm per hour), a heavy rain (23mm 

 per hour), and an excessive rain of cloud l)urst pro- 

 portion (43 mm per hour). The corresponding drop 

 size distributions were given by Best." 



In any rain the resulting total attenuation is the 

 sum of the gaseous (oxygen plus water vapor) and 

 eori)uscle or licpiid drop attenuation values. 



It is thus seen that for waves of 3 cm or shorter the 

 rain attenuation may become prohibitive, whereas the 

 gaseous attenuation loses its practical importance at 

 waves longer than about 3 cm. The attenuation of rain 

 computed in this report extends from 5 cm toward 

 longer waves. In the region A = 1.25 — 5 em, only 

 two attenuation values are available,^^ at 1.35 and 3 

 cm respectively. The dashed portions of the rain atten- 

 mition curves are thus extrapolations drawn through 

 the two computed points. The shape of these extra- 

 polated portions of the curves, in view of the decreas- 

 ing trend of the computed dielectric absorption values 

 with the wavelength,^'' seems to suggest that the rain 

 attenuation might level off or even decrease for waves 

 shorter than 1 cm. However, without a closer inves- 

 tigation of the raindrop absorption in this wavelength 

 region, no precise statement can be made on this 

 subject. 



As regards the normal atmospheric absorption of 

 microwaves, it may be mentioned that the o.xygen ab- 



